The present invention relates generally to micro-electromechanical devices and, more particularly, to micro-electromechanical thermal actuators such as the type used in ink jet devices and other liquid drop emitters.
Micro-electro mechanical systems (MEMS) are a relatively recent development. Such MEMS are being used as alternatives to conventional electro-mechanical devices as actuators, valves, and posititioners. Micro-electromechanical devices are potentially low cost, due to use of microelectronic fabrication techniques. Novel applications are also being discovered due to the small size scale of MEMS devices.
Many potential applications of MEMS technology utilize thermal actuation to provide the motion needed in such devices. For example, many actuators, valves and posititioners use thermal actuators for movement. In some applications the movement required is pulsed. For example, rapid displacement from a first position to a second, followed by restoration of the actuator to the first position, might be used to generate pressure pulses in a fluid or to advance a mechanism one unit of distance or rotation per actuation pulse. Drop-on-demand liquid drop emitters use discrete pressure pulses to eject discrete amounts of liquid from a nozzle.
Drop-on-demand (DOD) liquid emission devices have been known as ink printing devices in ink jet printing systems for many years. Early devices were based on piezoelectric actuators such as are disclosed by Kyser et al., in U.S. Pat. No. 3,946,398 and Stemme in U.S. Pat. No. 3,747,120. A currently popular form of ink jet printing, thermal ink jet (or xe2x80x9cbubble jetxe2x80x9d), uses electrically resistive heaters to generate vapor bubbles which cause drop emission, as is discussed by Hara et al., in U.S. Pat. No. 4,296,421.
Electrically resistive heater actuators have manufacturing cost advantages over piezoelectric actuators because they can be fabricated using well developed microelectronic processes. On the other hand, the thermal ink jet drop ejection mechanism requires the ink to have a vaporizable component, and locally raises ink temperatures well above the boiling point of this component. This temperature exposure places severe limits on the formulation of inks and other liquids that may be reliably emitted by thermal ink jet devices. Piezoelectrically actuated devices do not impose such severe limitations on the liquids that can be jetted because the liquid is mechanically pressurized.
The availability, cost, and technical performance improvements that have been realized by ink jet device suppliers have also engendered interest in the devices for other applications requiring micro-metering of liquids. These new applications include dispensing specialized chemicals for micro-analytic chemistry as disclosed by Pease et al., in U.S. Pat. No. 5,599,695; dispensing coating materials for electronic device manufacturing as disclosed by Naka et al., in U.S. Pat. No. 5,902,648; and for dispensing microdrops for medical inhalation therapy as disclosed by Psaros et al., in U.S. Pat. No. 5,771,882. Devices and methods capable of emitting, on demand, micron-sized drops of a broad range of liquids are needed for highest quality image printing, but also for emerging applications where liquid dispensing requires mono-dispersion of ultra small drops, accurate placement and timing, and minute increments.
A low cost approach to micro drop emission is needed which can be used with a broad range of liquid formulations. Apparatus and methods are needed which combine the advantages of microelectronic fabrication used for thermal ink jet with the liquid composition latitude available to piezo-electro-mechanical devices.
A DOD ink jet device which uses a thermo-mechanical actuator was disclosed by T. Kitahara in JP 2,030,543, filed Jul. 21, 1988. The actuator is configured as a bi-layer cantilever moveable within an ink jet chamber. The beam is heated by a resistor causing it to bend due to a mismatch in thermal expansion of the layers. The free end of the beam moves to pressurize the ink at the nozzle causing drop emission. Recently, disclosures of a similar thermo-mechanical DOD ink jet configuration have been made by K. Silverbrook in U.S. Pat. Nos. 6,067,797; 6,234,609; 6,239,821; and 6,243,113. Methods of manufacturing thermo-mechanical ink jet devices using microelectronic processes have been disclosed by K. Silverbrook in U.S. Pat. Nos. 6,254,793 and 6,274,056.
Thermo-mechanically actuated drop emitters are promising as low cost devices which can be mass produced using microelectronic materials and equipment and which allow operation with liquids that would be unreliable in a thermal ink jet device. However, operation of thermal actuator style drop emitters, at high drop repetition frequencies, requires careful attention to the effects of heat build-up. The drop generation event relies on creating a pressure impulse in the liquid at the nozzle. A significant rise in baseline temperature of the emitter device, and, especially, of the thermo-mechanical actuator itself, precludes system control of a portion of the available actuator displacement that can be achieved without exceeding maximum operating temperature limits of device materials and the working liquid itself. Apparatus and methods of operation for thermo-mechanical DOD emitters are needed which manage the effects of heat in the thermo-mechanical actuator so as to maximize the productivity of such devices.
A useful design for thermo-mechanical actuators is a cantilevered beam anchored at one end to the device structure with a free end that deflects perpendicular to the beam. The deflection is caused by setting up thermal expansion gradients in the beam in the perpendicular direction. Such expansion gradients may be caused by temperature gradients or by actual materials changes, layers, thru the beam. It is advantageous for pulsed thermal actuators to be able to establish the thermal expansion gradient quickly, and to dissipate it quickly as well, so that the actuator will restore to an initial position.
The repetition frequency of thermal actuations is important to the productivity of the devices that employ them. For example, the printing speed of a thermal actuator DOD ink jet printhead depends on the drop repetition frequency, which, in turn, depends on the time required to re-set the thermal actuator. Cantilevered element thermal actuators, which can be operated in a pulsed mode with rapid recovery, are needed in order to build systems that operate at high frequency and can be fabricated using EMS fabrication methods.
It is therefore an object of the present invention to provide a thermo-mechanical actuator which is operated in a pulsed mode and which resets quickly, allowing rapid repetion of the actuations.
It is also an object of the present invention to provide a liquid drop emitter which is actuated by a thermo-mechanical cantilever.
It is further an object of the present invention to provide a method of operating a thermo-mechanical actuator in an efficient manner such that repeated actuations have similar characteristics of motion.
The foregoing and numerous other features, objects and advantages of the present invention will become readily apparent upon a review of the detailed description, claims and drawings set forth herein. These features, objects and advantages are accomplished by constructing a thermal actuator for a micro-electromechanical device comprising a base element and a cantilevered element extending from the base element and normally residing at a first position before activation. The cantilevered element includes a barrier layer constructed of a low thermal conductivity material, bonded between a deflector layer and a restorer layer, both of which are constructed of materials having substantially equal coefficients of thermal expansion. The thermal actuator further comprises an apparatus adapted to apply a heat pulse directly to the deflector layer, causing a thermal expansion of the deflector layer relative to the restorer layer and deflection of the cantilevered element to a second position, followed by restoration of the cantilevered element to the first position as heat diffuses through the barrier layer to the restorer layer and the cantilevered element reaches a uniform temperature.
The present invention is particularly useful as a thermal actuator for liquid drop emitters used as printheads for DOD ink jet printing. In this preferred embodiment the thermal actuator resides in a liquid-filled chamber that includes a nozzle for ejecting liquid. The thermal actuator includes a cantilevered element extending from a wall of the chamber and a free end residing in a first position proximate to the nozzle. Application of a heat pulse to the cantilevered element causes deflection of the free end forcing liquid from the nozzle.